The world's two great superpowers were at war, their mighty fleets battling across much of the globe. Yet scientists from both countries sailed into the world's unfriendly waters in search of a scientific treasure: a transit of Venus across the face of the Sun.

The transit would take place in June of 1761. By watching it from several widely spaced locations, astronomers hoped to measure the distance to Venus, which in turn would give them the distance to the Sun -- a key "yardstick" in measuring all astronomical distances.

Diagram of the 1761 Venus transitSo working together, scientists from the superpowers -- Britain and France -- arranged several expeditions. Given the war, the weather, and other conditions, they were dangerous journeys.

One of the French expeditions, for example, sailed for the Indian Ocean. It was chased by the British fleet around the Cape of Good Hope, and was blocked from its intended destination by the war. The expedition did see the transit, but from a ship, where it was impossible to record the time, making the observations useless.

Another expedition headed for Siberia. But most of its instruments were smashed by bumpy roads before it even left France. Later, the expedition eventually had to sled across Siberia during winter.

One of the British expeditions was bombarded by a French ship just a day after leaving port. The leaders wanted to give up. But some unfriendly persuasion kept them going. More about that tomorrow.